The features of many peripheral sensory epithelia have been demonstrated to be labile to altered sensory inputs, however, the extent to which this is a global feature of the peripheral nervous system (PNS) has remained elusive. One way in which this can be studied is through investigations involving genetic mouse models for sensory hypofunction in the vestibular epithelia, since these peripheral structures are normally under constant sensory load throughout life. Here, it was investigated how altering vestibular sensory information to the inner ear epithelia (utricle, horizontal and superior cristae), using the otoferlin-null (Otof-/-) mouse may lead to presynaptic and/or postsynaptic changes in the expression of commonly expressed Ca2+ binding proteins oncomodulin (OCM, �- parvalbumin) or calretinin (CALB2). No alterations were observed in the presynaptic expression of OCM or CALB2, however, there was a marked heterogeneous decrease in postsynaptic CALB2 expression in the Otof-/- epithelia. These results suggest that some epithelial features (postsynaptic CALB2 expression) of the vestibular system are sensitive to the loss of OTOF and dynamic vestibular sensory inputs, while others (presynaptic OCM or CALB2 expression) are not. This may provide insight into how widespread the abilities of PNS sensory structures to adapt to changes in sensory information are, as well as how certain peripheral cellular features within the vestibular system are developed and maintained.

Ribbon synapses are one of the primary means of synaptic transmission within mammalian sensory systems. Loss of these synapses results in potential sensory dysfunction, as a result, their quantification is of critical importance to understanding the effects synaptic pathology may have on symptomatic sensory deficits. Immunohistochemistry is considered a reliable method to label these synapses, yet challenges arise when quantifying their distributions. In the vestibular periphery, low associations with standard postsynaptic markers and manual segmentation of these synapses have created significant barriers to our understanding of the relationship between changes in synapse density and its potential implications in pathology. The purpose of this study is to provide a novel method emphasizing rigorous analyses of immunolabeled ribbon synapses in the vestibular periphery, advancing our understanding of synapse distributions, and potential impacts various pathologies may have on these specialized structures.

Bats are highly agile animals by virtue of their flight and echolocation behaviors, and they are acknowledged to be “spatiotemporal integration specialists.” In order to avoid obstacles, scavenge food, and capture prey, bats must be able to navigate a three-dimensional space. In flight, bats must perform high speed navigation through obstacles using a blend of sensory modalities including vision and echolocation. While some bat species exhibit structural adaptations to their cochleae as an adaptation to the demands of echolocation, there is an absence architectural adaptation of the vestibular labyrinth that would accommodate the increased demands for high frequency head movement coding associated with agile movement. The present study tested the hypothesis that there may exist cellular adaptations within the vestibular epithelia consistent with increased capability for high fidelity head movement coding. This was accomplished using immunohistochemical analyses focusing primarily in the striolae of the utricles of Rousettus aegyptiacus and Mus musculus, regions bounded by the presence of calretinin (Calb2) positive calyces. Hair cell counts revealed increased proportions of type I hair cells (hair cells surrounded by calyces) in Rousettus when compared to Mus (72.6% � 3.5% and 57.2% � 2.7%, respectively, p<10-5). Additionally, Rousettus exhibits increases in the proportion of complex calyces in the striola when compared to Mus. Ocm, a calcium binding protein expressed in vestibular hair cells, is also observed to have expanded expression in Rousettus and Mus both in the striola as well as the extrastriolar regions. These novel findings indicate that vestibular epithelia of Rousettus exhibit cellular adaptations over Mus that are consistent with increases in head movement coding capability associated with agile movement in flight.